1. Field of the Invention
The present invention relates to a super-critical refrigerant cycle system in which pressure of refrigerant discharged from a refrigerant compressor is higher than the critical pressure of refrigerant. More particularly, the present invention relates to improvement of heat-exchange performance in a heat-pump water heater including a water-refrigerant heat exchanger where water to be used is heated by performing heat-exchange with high-pressure side refrigerant discharged from the refrigerant compressor.
2. Description of Related Art
As disclosed in JP-A-2001-82803, a conventional heat-pump water heater includes a water-refrigerant heat exchanger for heating water to be used by performing heat-exchange between the water and high-pressure side refrigerant discharged from a refrigerant compressor. As a heat source unit for heating the water, a super-critical heat pump cycle is used. In the super-critical heat pump cycle, carbon dioxide (CO2) is used as refrigerant, and pressure of refrigerant discharged from the refrigerant compressor is higher than the critical pressure of refrigerant. The super-critical heat pump cycle is constructed so that refrigerant discharged from the refrigerant compressor is returned to the refrigerant compressor through the water-refrigerant heat exchanger, an expansion valve, a refrigerant evaporator and an accumulator in this order. It is known that water heating performance of the super-critical heat pump cycle is improved by adding an internal heat exchanger thereto. The internal heat exchanger is for performing heat-exchange between refrigerant flowing out from the water-refrigerant heat exchanger and refrigerant flowing out from the refrigerant evaporator.
However, when the internal heat exchanger is added, the temperature of refrigerant discharged from the refrigerant compressor is abnormally increased, thereby extremely reducing lives of components of the heat pump cycle. Therefore, a heat-exchange amount of the internal heat exchanger is required to be controlled, and a dedicated component for controlling the heat-exchange amount of the internal heat exchanger is required to be added, thereby increasing production cost of the heat pump cycle.